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When does an airliner switch from using the tiller to the rudder?
When does an airliner switch from using the tiller to steer to the rudder on takeoff? Same for landing- when does the pilot switch from using the rudder to the tiller? What is this process? Once a pilot switches to the rudder on takeoff, is there a lock for the tiller? I'd imagine that it would be necessary to have some sort of system to lock the tiller so the pilot cannot accidentally hit it at high speed on the ground. If there is a locking mechanism, does it actually lock the tiller or does it simply disconnect the tiller from the nose wheel leaving the wheel free spinning so it can move as the rudder makes slight corrections while taking off/landing? Please expand as much as possible! Thanks.

I flew for two 747 carriers that never bought new aircraft, and thus this answer applies to 747-100/200 aircraft as originally manufactured for a number of different airlines.
The tiller is active when the aircraft is on the ground and the nose gear is compressed regardless of groundspeed as I remember.
I've never heard of a pilot accidentally hitting it at high speed on the ground, but if they did it would scrub the front tires. It wouldn't appreciably turn the airplane.
Pilots did have to be wary of having their hand within the travel range of the tiller when a tug is being hooked up. Ground handlers would sometimes work the nose gear back and forth a bit, and that action is reflected in tiller movement. If they do it quickly, the tiller will snap quickly and a hand in the way will get hurt.
The nose gear is relatively weak compared to the rudder, the brakes, or the engines insofar as its ability to turn the aircraft. To make turns off a runway onto a taxiway that is 90° to the runway, you usually aim for a 10 knots or less groundspeed. You can do a little more depending on the runway surface and what your center of gravity is, but sometimes 10 knots can be too much. I once skidded the nose gear badly at Jeddah, Saudi Arabia at midday in the summer when I failed to account for the fact that hot asphalt is slippery.
To illustrate how poor tires are in preventing sideways scrubbing, consider the 747 body gear. 747s have body gear steering for tight turns, but you can dispatch the aircraft with inoperative body gear steering. You can still make tight turns, but not quite as tight, but you do it with engine power and just scrub the body gear around, all 8 tires with a lot of weight on them. You might have difficult doing that, though, if you c.g. is greater than 26.6% mac. That's where the wing gear is, and if you're aft of that (limit is 33% to 37% typically) you're got a lot more weight on the body gear. Body gear steering is disarmed for takeoff and landing though.
Barring a strong crosswind, the usual takeoff procedure was to not use the tiller unless you were doing a rolling takeoff, starting the power up before being aligned with the runway. Once aligned and moving the rudder will do it. Even in a strong crosswind, you can forget the tiller very soon in the takeoff roll.
On landing, no use of the tiller until down to taxi speed except maybe a little use for a strong crosswind just before reaching taxi speed.
Boeing offered a rudder pedal-nose gear steering link, but almost all 747s I flew did not have that. I flew a couple of airplanes that had it. It helped in a crosswind.

Depending on the manufacture of the aircraft (Boeing or Douglas) the use of the tiller or nose wheel steering is different. Douglas aircraft, like the DC-8 and DC-9, had limited nose wheel steering (15 deg) through the rudder peddles. These aircraft could be taxied to a limited turn radius with the rudder peddles as they were linked into the nose wheel steering mechanism. However, turns of more than 15 degrees needed the tiller (nose steering wheel) to accomplish the greater degree turn. On take-off the nose wheel steering was not normally used once the aircraft was aligned with the runway as the 15 deg. limit was enough to control the aircraft until positive rudder control was attained during acceleration. If the take-off was being made by the F/O, he had control of the aircraft from runway alignment through the entire take-off procedure. The Captain would only place his hand lightly on the nose wheel steering until positive rudder control was attained and the "80K call" had been made. This was done in the event of an engine failure below V1 and an abort was necessary. Boeing, like a B-707, had no such linkage. Moving the rudder did absolutely nothing until positive rudder authority was gained on the take-off roll; usually somewhere around 60K. Using the nose steering was necessary until the rudder became effective and, failure to do so, would most like put the aircraft off the runway unless the wind was calm. If the co-pilot was making the take-off in the B-707, he would call "I have control" once he felt the rudder could steer the aircraft and the Captain would then rest his hand on the tiller until the 80K call was made. When taxing a Douglas aircraft, the Captain could take his hands off the tiller (nose wheel steering) if needed, to accomplish a task that required both hands. Not so, in the Boeing. If left unattended even for a short while, the Boeing would invariably head off the taxiway until the Captain made a fast grab for the tiller to correct the wayward aircraft.  One last note: Three engine ferries on the DC-8 were relative easy to accomplish because of the nose wheel steering through the rudder. However, the B-707 required a well coordinated crew effort and many B-707s were lost on a three engine ferry attempt due to poor control transfer.

All 737 models have nosewheel steering interconnect with the rudder pedals.

Good addition! I never flew the B-737 but nosewheel steering interconnect to the rudder pedals, obviously, was an improvement over the B-707.

Basic rule is..
Tiller for Taxiway
Rudder pedals for Runway.

SOPs in my company prevent the use of tiller above 20kts.(it’s a limitation)
Note that on our fleet rudder pedals can deflect the nosewheel up to 6deg (compared to 75deg on tiler), reducing linearly to 0deg at 130kts (Airbus 320), so there is still some limited turning authority until the rudder takes over.


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What aircraft manufacturer uses an HSI with DME, VOR, and G/S in one Instrument?
Which aircraft manufacturer uses a horizontal situation indicator (HSI) with DME, Glide Slope, VOR, and ADF in one Instrument?

The Garmin G1000 can do that... but I'm thinking you are wanting the old "steam" type instrument?

Likewise, the Collins Proline 21 (and I assume most other Glass cockpits) can do the same with multiple pointers on the PFD or MFD.

source: Stack Exchange

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Why do some airplanes fly at a very low speed (like this one) at a speed of 433 km/h (234 knots) at a whopping high altitude of 26,500 ft?

Okay. So I'am just having a doubt on this flight (BA359) flying from Lyon, France en-route to London Heathrow. It is flying at a way low speed of 433 km/h (234 knots) in which i think is a bit weird. It is quite far from London Heathrow before landing. But come on, the head wind is not way too much. I mean if it is way too much, the airplane will fly at TAS 700 km/h ( GS 950 km/h) corrected for a 250 km/h head wind/jet stream.
Or maybe there is an altitude restriction or instruction to fly at holding speed to finally hold at Biggin or Ockham holding stacks due to heavy inboud traffic? I'am just curious.

Ground speed is not the same as airspeed.  That plane may be facing a strong headwind.

Was the airplane climbing?

433km/h seems really low, but keep in mind that flightradar is far from perfect. It is not uncommon that altitude, speed or position are far off, and sometimes, you see sudden jumps to more correct values when flight radar gets new data.
You can also check other sites like flightaware for particular fligt numbers, where you get a graph of altitude and speed, and can easily play back a given flight.
You didn't post a flight date, but the flights of the last few days have a speed of 600-700km/h at that position. This is below cruise speed, but the aircraft already start to reduce altitude and speed when leaving France.

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"Total time away from gate", "Longest time away from gate" - what does this mean?
In the US, airlines or airports report two figures regarding canceled flights:

the total time away from the gate and the single longest time away from the gate.

and regarding diverted flights:

the total time on the tarmac at the diversion airport and the longest single time on the tarmac.

I don't understand this distinction, nor the individuall phrases.

What is a "single time"? A single time period out of multiple ones?
Why would a plane be away from the gate multiple times? i.e. leave the gate, return to the/a gate, leave again and return again?
Why would a diverted plane be on the tarmac multiple times at the same airport?

The US regulations have the tarmac delay rules, which apply to extended delays where passengers are left sitting on the tarmac. This followed a variety of incidents where passengers were kept on board non-moving aircraft for long periods of time, sometimes without access to food and water. The rules are a bit complicated with various exceptions, but broadly:

How long can an airline keep me on a DEPARTING flight before the
  airline is required to start moving the airplane to a location where
  passengers can safely get off?
For flights departing from a U.S. airport, airlines are required to begin to move the airplane to a location where passengers can
  safely get off before 3 hours for domestic flights and 4 hours for
  international flights.

So yes, a flight can potentially have multiple tarmac delays. For example, a flight could potentially leave the gate on time, sit on the tarmac for 2.5 hours, return to the gate (to allow passengers a chance to get off, as is required), leave the gate, wait another hour, be cancelled, and return to the gate. The airline would report both all the time the plane was away from the gate (2.5 + 1 = 3.5 hours) and the longest period the plane was away from the gate (2.5 hours).

What is a "single time"? A single time period out of multiple ones?

Yes. If the flight sits waiting on the tarmac for more than one period, the "single time" is whichever period was the longest.

Why would a plane be away from the gate multiple times? i.e. leave the
  gate, return to the/a gate, leave again and return again?

It could happen for a number of reasons. One would be that the plane returned to the gate to comply with the rule and allow passengers an opportunity to get off after 3-4 hours delay. The plane might return to the gate for a maintenance issue. They could be waiting out bad weather or a situation where an airport is not accepting flights. Most airlines seek to avoid this, but it can happen, and the reporting requirement is intended to capture data about how long it takes when it does.

Why would a diverted plane be on the tarmac multiple times at the same airport?

The same reasons. I had this happen once due to a tornado. The flight diverted, we went to a gate, we went back to the tarmac to wait for better weather, the flight was cancelled, so we went back to a gate again. It's an unusual situation, but it can happen.

Planes can be stuck on the tarmac for long enough periods that they no longer have enough fuel to make it to their destination safely, so they have to return to a gate to get more--and as soon as they're done, they typically get kicked off that gate because other planes have the same problem, so they go back out to sit on the tarmac. Of course, while they're at the gate they usually load more food and drinks as well, which are also often running low if you've been sitting that long.
The reason this is tracked specially is that when the plane returns to the gate, passengers are allowed to deplane.  So, a single time away from the gate is how long passengers are "trapped" on the plane, which is psychologically and politically different from how late the flight leaves.
If a plane is diverted to an alternate airport, the same scenarios play out as above at the origin airport with the same reasons.

Huh. I didn't realize the fuel consumption at rest really that significant. Also, are you sure this is the single and only reason for returning to the gate?

The ground burn rate isn't significant, but it's not zero either.  IFR flights must have enough fuel at takeoff to fly to their destination and then their alternate plus 45 minutes for variations in routing/winds.  Airlines may load some extra for expected ground delays, but too much means burning more fuel to carry extra weight around, which adds up fast for thousands of flights per day.  They'd rather occasionally return to the gate for a top-up--and to give angry passengers a chance to deplane.

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On a axial flow turbo fan or turbo jet engine what comes first the rotor or the stator?
I've been let to understand that one stage consists of a row of rotors and stators. But I'm not sure which comes first? Does the front fan blades make the first part of the stage (rotor) with a stator section behind? And so on.. Or on a fan engine with a booster the first stage is the stator and then rotor (after the fan blades).
Secondly do all engines have inlet guide vanes? I'm assuming not or we would physically see them in front of the the big fan engines or do they appear later on.. Maybe in the HP section?
Hope this makes sense and any images with an explanation would be appreciated.
Thanks

There are two aspects to consider here:

The aerodynamic principle:
The simple aerodynamic principle requires a compressor stage to have first a rotor followed by a stator.
The constructive aspects:
Constructive aspects to be considered are:
a) Pre-rotation of air before the rotor: in order to reduce the relative speed of the airflow on the rotor blades (to avoid the shock waves of supersonic airflow) a set of stators can be placed in front of the rotor; this will rotate the air towards the direction of rotation, reducing by this the relative speed of the airflow on the rotor blades; even in such cases a stator stage is required after the rotor.
b) Bearing location: some designs put the bearing behind the rotor, other designs put the bearing in front of the rotor; in such case the bearing before the rotor needs to be supported by a system of vanes.

Consequently, many smaller diameter (or high speed military design) engines will take advantage of a) and b) above and have a stator in front of the first stage compressor.
To answer the same questions on turbines, there the aerodynamic principle requires to have first a set of stators and then a rotor.

In an axial compressor, the air is compressed by grabbing it with a moving blade and squeezing it against a fixed surface. This is a very simplified representation, of course, but it gets the idea across.
For this reason, the rotor goes first, the stator follows.
Inlet guide vanes are only present in some engines. They do not replace a stator, only make the rotor more efficient, but add their own drag.

Rotor, then stator, in as many subsequent pairs as the engine is designed to contain.
Source: https://web.mit.edu/16.unified/www/FALL/thermodynamics/notes/node92.html

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On a centrifugal compressor impeller what is the difference between the radial vanes and rotating guide vanes?
I thought the radial vanes were a machined part of the impeller that just force the airstream radially? And that another name was rotating guide vanes as they are rotating with the impeller disc.. But the picture from a textbook I have is referring to them as 2 different things? Or have i misunderstood something?
Thanks

The radial vanes are the main compression channels, where air gets squeezed into 5 - 10 bar and gets slung out of the tip traveling at 500 m/s.
The rotating guide vanes scoop the incoming air into the impeller vanes, kind of like a pre-whirl tower. They’re at the inside diameter and travel at much lower linear speed, subject to lower centrifugal force.

This picture was used in an earlier answer, and illustrates the different roles of the channels and vanes. As the pressure ratio on the horizontal axis increases, we can observe the following:

Efficiency decreases: losses are higher for higher single stage pressure ratios, which is the main reason that commercial aircraft turbine engines use axial compressors.
Impeller height increases. The curved shape allows for a more gradual change of direction of the airflow: inflow is in axial direction, outflow in radial direction. A large potential source for efficiency loss.
Inside vane shape points more and more in radial direction: it scoops the airflow into the impeller, and pre-whirls into the radial direction that will be imposed by the impeller.

The right-most impeller combines an axial stage with a centrifugal stage. In an axial compressor the rotor is followed by a stator, in our axial-centrifugal combination there must be no stator because we want whirling flow now.

With advances in production technology, this has resulted in impeller shapes with double curved inlet vanes, such as above. The inner bits of the impeller are subject to much lower stresses and may be shaped much more funkily than the impeller outer diameter.

I'm still confused.. If the radial vanes are the machined part of the impeller, where are the rotating guide vanes mounted? And how do they rotate at a lower speed than the impeller? Do you have an image that shows them both clearly?

Oh wait is the rotating guide vane and radial guide vane and impeller just one assembly? Impeller being the disc or hub, rotating guide vanes being the vanes you can see mounted on the impeller or machined part of.. and the radial vanes are the channels beneath the vanes that the air travels up towards the diffuser stage?

Yes exactly. The guide vanes rotate at the same speed, the linear speed is lower.

The "guide vanes" may be the taller ones closer to the center of rotation.  The next sentence after the OP highlighted one tells more.


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How to identify different pipes / hoses on aircraft engines?
When looking at an aircraft engine (without any cowlings) is there an easy way of identifying what pipes are for fuel, air, oil, hydraulics etc..? Maybe certain fluid pipes are different size diameters or do you just have to know the engine system and components they are providing to be able to tell?

You can guess what most of the tubing goes to if you know a few of the parts under the cowl and have a general understanding of how engines work. You can also get a good idea of what each part is by studying the parts catalog for the aircraft. You can view the Parts Manual for my Cherokee and an Arrow that I fly.
Piper Arrow Six Cylinder Continental

Let’s start with easy ones to identify. You should be able to find the tailpipe rather easily.  Notice in the picture of the Arrow that there are a lot of big orange tubes. That’s called scat tubing and it is used to move air around the engine compartment. In most singles, there is a shroud around the muffler that has orange hoses coming out of it. The ones going into the firewall provide cabin heat and defrost. The ones going to the carburetor provide carb heat to prevent icing.
Notice the rusty pipes coming out of the bottom of the engine. Those are the exhaust pipes and it’s hard to see in this picture, but they connect to the muffler. The big black pipe at the top of the engine is connected to the air filter and is the air intake. The clean pipes along the top of the engine are the air intake tubes. Most fuel-injected engines have the air intakes on the top of the engine.
Fuel injected engines also have small lines that put fuel directly into each cylinder. The line entering the "spider" at the top is a fuel line and you can trace it back to the fuel pump.

The brown hose in the picture below going to the spider is a fuel line.

Cherokee Lycoming Four Cylinder Engine

Compare this to the carbureted engine in the Cherokee. The more rusty pipes are the exhaust and the cleaner blueish pipes come from the carburetor with air and fuel.
If you can find the vacuum pump (the dark item to the right of the oil filter with the red bands) you can identify the hose going to the attitude indicator and heading indicator. There’s usually a filter attached to the hoses and a line leading to the vacuum gauge—although sometimes that is inside the cabin. The big open-ended hose is the exhaust for the pump.

Here’s another four cylinder where you can see the intake and exhaust. Notice the silver lines with the blue fitting. Those are primer lines. At the bottom left is an oil cooler. There is scat tubing bringing air to the cooler and two lines going in and out.

Oil coolers are often in the front of the cowl like this one. If there are lines attached to something that looks like this, they are oil lines.

This is a different method of plumbing carb heat. Rather than coming off the muffler hot air comes directly from the engine.

Here’s a picture with lots of hoses.

The two small hoses at the bottom of the picture are attached to the oil cooler. If you follow them back you’ll find the oil pump.
You can see the carburetor with a scat tube coming from the air filter and one on the back side coming from the muffler shroud. The two small hoses going into the carb are fuel lines.
The vertical silver tube running from the top of the picture, down past the muffler, and out the bottom is the crankcase breather line.
The box in the bottom right-hand corner has the electric fuel pump (boost pump). The line coming out of it is a fuel line. The silver tube in the box goes to the gascolator.
You should also see an oil line going through the firewall that measures oil pressure and one for fuel pressure but I don’t have pictures that show them.

Air pipes are always going to be a lot larger diameter than everything else. But beyond that I don't know any general rules. Oil fuel and hydraulic lines can look pretty similar. If you narrow it down to one specific engine (e.g. a cfm56-7) someone might be able to give some heuristics. But just engines in general is pretty broad.

On GA airplanes you will usually see black braided hose made to MIL-H-8794 (several thousand PSI burst pressure) used on anything that carries fluids under significant pressure (engine oil, fuel, brake lines, and coolant on an engine with liquid cooling).  The most common is Aeroquip 303 and Stratoflex 111, which have a black braided cloth kind of outer surface texture (with the hose model printed along it in yellow or white).
There are variations of the high pressure rubber hose with an extra stainless braided layer on the outside (Stratoflex 156 and Aeroquip 601), and also braided stainless over a Teflon inner hose (Stratoflex 124 and Aeroquip 666).
On transport airplanes and maybe on some GA airplanes there will be sticky wrap-around printed identification tape that says FUEL, OIL etc.  Absent that, you have to go by the components that the hose runs to (like a hose going to an oil filter or cooler).
As a general rule on a light aircraft, hose that is fairly thick, 3/4 to 1 inch thick in an engine compartment is probably engine oil and smaller hoses of say 1/2 or 5/8 inch diameter are fuel or brakes.
Beyond that, if black rubber hose has a smooth texture, it will be low pressure hose used for vent lines, or instrument vaccum pump lines, like Aeroquip 306.
So to summarize, on light aircraft:

Fatter hose up to 1 inch with a braided external texture, either black or stainless steel, is usually engine oil or coolant if the engine uses it.
Thinner hose, say up to 5/8 inch with a braided external texture, either black or stainless steel, usually fuel or hydraulics (brakes).
Fatter black hose with a smooth outside texture will usually be something with low pressure air in it like a crankcase vent.
Thinner black hose with a smooth outside texture will usually something with low pressure air or suction like a vacuum line running from a vacuum pump to the firewall, or a manifold pressure line going from the engine to the firewall.

One problem is, on fuel, oil and hydraulic lines under the cowling, the hoses are usually nowadays covered with protective firesleeve, which hides what is under, so often you are stuck in the end with identifying it by what is at the ends regardless.


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How do the wings connect to the centre wing box?
First of all does the centre wing box form part of the fuselage as it's in the centre of the fuselage? Secondly how does the wing spars connect to this centre wing box to hold everything together?

Are you asking about a particular type? Seems to me this could vary widely among different aircraft.

Transport airplanes, in general, use one-piece wings with no joints between sections. The "wing box" is the central structural element for the entire wing, made up of front and back spar webs, and upper and lower planks, which make the 4 sides of the box.  Leading edges and trailing edges/control surfaces are tacked on to the front and back walls of the box.
The upper and lower planks (machined skins) are the primary structural members in bending, supported by the spar webs to resist shear loads and the wing ribs to support the planks themselves against buckling.  You could say that the entire upper and lower skins or planks are the spar caps.
The "center wing box" is the middle section of the overall wing box, but that just describes the center part of a very long box.  The fuselage will simply sit on top of it, with a square cutout in the bottom to fit over the wing, with fairings all around the connection to make an nice external transition for the air flow.
The fuselage will be integrated into the wing box structurally with some sort of connecting links joining fuselage frames to the box, and may also have fuselage skins attached directly to the wing box.  Sometimes the cutout in the fuselage has its own pressure bulkhead forming a box outside the wing box and you can simply unbolt the fuselage from the wing and lift it off, and sometimes the wing box itself is made part of the pressure hull, requiring lots of rivets to be drilled out to separate them.
If the airplane has wings that attach to each side of the fuselage with big spar fittings and bolts, the center wing box will be an integrated part of the fuselage, what you would call a structural pass-through.


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On a large transport aircraft where exactly is the Forward pressure bulkhead?
Is the Forward pressure bulkhead in front of the pilots.. I.e when you lift the Radome for maintenance is that the Forward pressure bulkhead you can see? Some images or a diagram would be great.

It's located  in the aircrafts radome (nose) where the weather radar is located. It's to keep the pressurized air from exiting the fuselage.

It doesn't always have to be there - on the A380 it's slightly further back than that and is a complex structure which involves half of the front wheel well - they build a further structure up to actually make the nose, but the pressure bulkhead is further aft.

@Moo that picture is strangely ominous, yet friendly...


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In aircraft air conditioning systems what are the differences between the air cycle machine system and the displacement blower type system?
I have seen reference to both these different air conditioning systems but can't understand what the key differences are and where each system would likely be found.. I.e is one system preferable over another system on a particular aircraft type?


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Maximum Altitude/Minimum Range limits of ATC PSR
I am trying to get an understanding of the ability of an air traffic control primary radar such as an ATCR-33 to track high flying targets (above 40,000 feet) at relatively close ranges (less than 15 nm horizontal distance from the radar).  An alternative way of asking the question is what is the maximum 'beam elevation' (I think that that is the correct term) for a radar like an ATCR-33?

This paper seems to show beam elevation from 0 to 40 degrees, dropping off after that.
See slides 10 and 15.

Thank you very much for that presentation, that's enormously helpful.


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Please identify this unusual jump plane
Can anyone identify this aircraft:
This plane was used in a horrible skydiving attempt where the person in the back attempted to climb out but was unable. As a skydiver, this made me very uncomfortable. I am guessing there is a large tail on this aircraft that would have made exiting fatal. The full video is here:

You sure that video is not fake?

The plane appears to be real.

I guess I would just like to believe that no one would stick their hand into the engine intake of an airborne fighter jet ...

The camera wobble looks weird to me. A camera fixed to the wing shouldn't move around like that. But it could be a result of image stabilization

The camera could have been a 360 camera, but we're only seeing a narrow field of the view (which is kept pointing the same way relative to the horizon by stabilization). I've seen this done a lot with RC plane videos.

That is an Aero L-29 Delfin

Source: onespotter, photographer/copyright: Evgeniy Lebedev

Good job. I spent 40 minutes trying to identify it without any success!

@ThatCoolCoder I got lucky. My first guess was something similar to the hawker hunter. Then I noticed the Russian flag on the nose and the name Peaktopw (?). Putting "peaktopw jet" into google image search gave one image with a jet. This one :-) . The aircraft is part of the Reactors Jetteam (https://www.youtube.com/watch?v=VRndI5CVqNo)

@DeltaLima: I'm not Russian, but the word 'peaktop' is 'reactor' written in Russian. P in cyrillic alphabet represents 'R' in latin alphabet. Also, 'K' is used for hard 'C' because in the cyrillic alphabet, 'C' is the equivalent of 'S'. The w at the end is I guess some representation of another cyrillic character which looks like 'bl'. That one is a bit tricky to explain, as it's not actually a letter. Look it up on wikipedia if you want to go down the rabbit hole :-).

@Gaston thank you for the explanation. It makes sense to me now. Went down the Ы-hole this morning already :-)

Awesome job. I am going to do some research and see if anyone has ever successfully jumped out of one. It does not look possible from my perspective.

@SDH I guess the backseater realised too that it wasn't looking positive when he was upside down hanging from the cockpit with his legs still inside and his hand in the jet intake while the aircraft was vibrating from his turbulence. One-of-a-kind type of experience.

@Gaston the word is "реакторы" - "reactors" in English. The last letter is actually a real letter even with its own corresponding sound (unlike similarly looking "Ь"). In this particular case it makes a plural  noun from a singular "реактор", similarly to "-s" in English.

@AlexeiLevenkov Sorry my bad. Thanks for the clarification, yes I mixed it up with 'Ь'


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Difficulties of landing an airliner without instruments
I just heard the communications from an Embraer E170 that lost all instruments after takeoff and had to return to the airport.
Brickyard E170
Can someone explain the procedure from the pilot's perspective in this kind of scenario? Are they essentially landing the plane by feel?

It is a little early to start speculating, but it is highly unlikely they had no standby instruments.

“Landing the plane by feel?” No, they can fly by looking outside and seeing where they’re going. Visually. Unless visibility is very poor, pilots take manual and visual control quite early on in the approach. And when doing that, they can’t but look outside. That’s normal. And the video shows the plane landing- sky conditions were fair, so no issues in just “eyeballing

It's most likely they lost one of the primary flight displays, or maybe both primary flight displays through some kind of one in a billion electrical coincidence, and were on the standby attitude indicator, which also provides airspeed and altitude.
If one of the primary flight displays blanked out, whomever has the working display will take over as Pilot Flying, and transfer the Flight Director (and hence the autopilot) to that display if it was linked to the side that failed.  With that kind of failure they will certainly request a turn-back immediately, but the return and landing will be pretty routine if one pilot has working instrumentation.
If both displays blanked out for some reason (extremely improbable) they would be on the standby instrument, the little mini attitude indicator in the middle, hand flying, and if they were in the clouds, the stress level would go up quite a lot until they were out of the cloud base and could see the ground, because it's a bit of a challenge to hand fly in IMC with that little attitude indicator in the middle of the panel.
Normally there's a ILS/LOC input to the standby att indicator, so you can fly a "raw data" (raw data means just glideslope and localizer indicator needles, no Flight Director) instrument approach (I've done it in the simulator - takes intense concentration and crew coordination).
If they were visual while all this was happening, the stress level goes way down because in a jet like that, all you really need is airspeed in visual conditions (altitude is nice to have, but you can see the ground so you can more or less maintain a safe altitude and you will already know from experience what pitch attitudes to use, and ATC can correct you if you drift too much up or down).
If you're truly proficient in the type, you should be able to fly in visual conditions without airspeed indication as well, and make it down in one piece, using just pitch attitude and thrust settings you normally set in the various configurations.
For example, flying final approach with full flaps and gear down, the N1 fan speed will always be within some small range, say 66-70% depending on weight. Just set thrust at say 66% if you're light, or 70% in the case of a departure where you're returning still pretty heavy, and follow the glide slope, and the speed will take care of itself, within the ballpark at least, assuming no windshear or other additional excitement.
For really extreme cases where you lose all three airspeed inputs (which is actually a pitot static condition caused by ice crystals at high altitude that block all 3 pitot probes - probability wise, simultaneous triple failures can only come from a single external source) the manufacturers provide pitch/power tables in the cockpit handbook (Quick Reference Handbook) that allow you to achieve ballpark speed profiles on pitch and thrust alone for all the normal configurations and weight ranges. A highly skilled pilot with long experience on type could get by without them, but they cater to the complete range of skill levels.

Not being familiar with all of the systems and terminology, I assumed when they said 'all instruments down' that they lost all instrumentation in the cockpit. Thanks for the great explanation.

Additionally, the QRH will typically contain a flight with unreliable airspeed indication section, where you can look up what pitch and thrust setting to use for maintaining a safe airspeed.

Oops I forgot to mention that, although that one's for loss of all 3 pitot systems, not displays blanking out.  The one created for the CRJ was done not that long ago, and was a monstrosity with pages and pages of cautions and advice b/c all the internal stake holders couldn't resist insisting on this or that. The operators criticised it bitterly. IIRC Boeing and Airbus unreliable AS procedures took up only a couple of pages of QRH, mostly the pitch/power tables.


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How does the fuselage handle the different loads?
I have been trying to find out how the loads experienced by a fuselage are transferred between structural members (bulkheads, formers, frame assemblies, skin, longerons, stringers).
I understand that the following loads are typically experienced by the fuselage:

empennage loads due to trim, manoeuvring, turbulence and gusts
pressure loads due to cabin pressurisation
landing gear loads due to landing impact, taxiing and ground manoeuvring
loads due to the weight of passengers and cargo

What I don't understand is how are these loads handled by the fuselage structure. I would really appreciate if someone could help explain the concept to me.

Not sure this is what you're looking for, but if you're looking for a simple big picture analogy, try this:
Think of the fuselage as a very thin walled compressed air cylinder resting on an airbag at about it's mid point, filled with sandbags that are offset toward one end, enough so that the cylinder wants to tip over on that end.  All the way at the other end you add more sandbags (to represent the tail pushing down) until the tube balances out.  Your fuselage will be basically a tubular beam balancing on the air bag with the bottom half in compression and the top half in tension, and the largest compression load will be just on each side of the air bag, but mostly on the "tail" side because the moment arm is longer.
Almost all of the loads of the fuselage pass through the skin, the surface of the cylinder.  The frames and stringers are mostly to keep the "paper thin" tube from buckling or collapsing.
As a compressed air cylinder, pressurization loads are also absorbed by the skin in tension, like a balloon.  Usually at the aft end there is a spherical bulkhead to absorb the pressure while keeping the skins of the bulkhead (mostly) in tension.  Sometimes this bulkhead is flat, usually when there are tail mounted engines that require a support beam running across, which can cause many headaches crack wise (on the RJs, a long sad story).  The front end of the cylinder narrows down to a small flat bulkhead at the front of the cockpit, which being small isn't too much of a problem being flat.
The fuselage usually has a big cutout on bottom to accommodate the wing box
and landing gear, which puts this great big notch in the structure right at the point in the cylinder where the compression buckling loads are highest.  The wing box itself may form part of the fuselage structure there, but farther back the cutout usually continues, to make space for the landing gear.  There is usually a massive beam, called a keel beam, to bridge the cutout aft of the wing box and absorb the compression loads along that section.
As a tube that is trying to sag forward and aft of the wing, there is also a force that wants to squash the tube flat at the peak bending point, resisted by the frames.  There are usually several very heavy frames at this mid section to support the tube where this force is highest, and to tie the wing box into the fuselage structure.
The highest loads on the fuselage are usually compression buckling stress just aft of the rear spar wing attachments when in flight, or the gear attachments when on the ground, and are highest on landing touchdown.  When aircraft have broken fuselages on hard landings (Boeing did it on their company 717 -formerly DC-9 - on hard landing testing and an Embraer 145 broke its back on landing in Brazil about 15 years ago - they did an excellent job keeping it out of the media) they fail in compression buckling right at that stress peak point.
I don't want to oversimplify, and there are lots of other stresses, like from the floor beams and such, but that's kind of the big picture.

Thanks. I think bending compression describes it, if by that you mean the force that makes a pipe collapse if you bend it without a mandrel to support the sides.

This is a pretty broad topic and can encompass several college level textbooks on aerospace structural mechanics, but sufficed to say, loads are transferred between members at joints and fall into four basic categories:

Axial loads, i.e., tension and compression loading.
Shear loads where one member tries to slide past another at their contact points.
Torsion loads where one member applies a twisting action to another member.
Bending loads - where on member attempts to bend or flex another.

Aside from this I’m going to need specific places in a specific aircraft where one would like to do a structural analysis.


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What's the name for the structure that helps stop the nose wheel in Boeing 747-400?
From my understanding the nose wheel of a Boeing 747 does not have brakes, and as such when it is retracted it is still spinning. However, there are two "plates" that make contact with the wheels and slow them down, eventually stopping them.
I am curious as to whether someone could provide some more information about this structure, especially the name.
See the image below:

Thank you!

They are called Snubbers or scuff plates. Spinning tires rub against them on retraction.


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How did the windshield technology evolve?
I would really appreciate it if someone could please explain the differences and similarities between the cockpit windows of a Boeing 747-400 and Douglas DC-3.
I am just trying to figure out how the technology would have evolved, what aspects would have remained and what would have changed. Literally just a brief comparison - nothing technical.
I have been unable to find information specific to the two models. I have found out the basic information with regards to a cockpit window, but I am unsure as to whether it applies for a Douglas DC-3 as well.

Kind of an extreme comparison no?  It's like the difference between the the windshield of an armoured car and a VW Beetle.

I am just trying to figure out how the technology would have evolved, what aspects would have remained and what would have changed. Literally just a brief comparison - nothing technical.

The two biggest differences both relate to their widely different cruise speeds: the 747 at 500kt and the DC3 at 180kt. The 747 is also pressurized and the DC3 is not.
The 747 is curved for aerodynamics.

It is also much stronger.

PPG Cockpit Windows
The DC3 windshield is weaker and flat. There are two major versions of the DC3, one with a wide flat pane and another that splits it two, though with no attempt at airflow improvements. It seems the smaller pane of the split version could be opened. It also apparently leaked like a sieve.

Birdstrike protection testing was mandated after the DC3 went out of production.

Section 25.775 requires windshields and their supporting structure withstand, without penetration, impact with a four-pound bird at VC (design cruising speed) at sea level. This regulation has been in place and is unchanged since part 25 was introduced in 1965.

You have undoubtedly heard of the NASA chicken gun, a compressed air device for accomplishing this test. Used by the British it on a train, it caused an extraordinary amount of damage to the cockpit being tested and even went through the rear wall into the passenger compartment. When enquiring of NASA as to the cause, NASA replied "Defrost the chicken".)
Mythbusters chicken gun


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Why are there so many aircraft designs which look generally profoundly similar?
The F-86, mig-15, Dassault Ouragan
The b-1 and tu-160
Concorde, tu-144, 2707
The low wing and T tail with two engines at the back. (CRJ, dc9)
The 'standard' shape with the mid aircraft wings, and vertical and horizontal stabilizers at the back, used from the E170 to the a380.
And while many general aviation craft are low winged or differ from a high wing tricycle single engine design, most seem to stick with that formula as well.
For some of the odder shapes, the question is 'industrial espionage?' but in the greater context, is it simple convergent evolution in that there are only so many shapes which are functional and useful so most things take on that general shape?

related (but also unanswered): https://aviation.stackexchange.com/questions/32405/is-there-a-term-for-the-general-design-used-in-most-modern-fighters

It is generally considered polite to wait for at least 24 hours before selecting an answer to allow people in every timezone to have time to answer.

If you go to a comprehensive aviation museum, such as Boeing Museum of Flight, and look at the models between about WW1 and the advent of Space Flight, you'll see MANY strange, innovative, experimental designs that put wings, engines, tails, and even fuselages in interesting configurations.  But in the end, only certain designs work out the best.  Its an evolution of engineering design: selection and survival of the best.

It’s an interesting question.  Aviation historian Bill Sweetman wrote about just that in an Air & Space article called How the 747 Got Its Hump.  The answer to the question is quite long and specific to the aircraft type and use but the short and sweet answer is that airplanes are a great compromise between a number of factors, including mission profile, aerodynamics, structures, propulsion, avionics, manufacturability, realities and practicalities of product use, and on a few occasions customer insistence and industrial espionage.  See illustration below.

It just works out that, when all the factors above are taken into account, the designs you describe tend to be the best all around solutions for particular aircraft sizes, costs, payloads, mission profiles, safety, etc.  Like any other artifact made by humans, aircraft are but single answers to open ended design problems; a compromise between all of these competing requirements into a tangible product.  No one answer is 'correct' per se, but these kinds of aircraft designs seem to be the most optimal and every single design team when confronted with the requirements and design constraints create much the same thing.

Thank you, so it does seem to be 'convergent evolution' based on responses to physics and current materials science, vs cost and (complicated formula involving all the above, plus what the person doing the end yes/no decision liked as a child)
Military aircraft are mildly easier to understand edge cases with due to the lack of a lot of the (hair,??,profit!) equation.

Wow, that is a great Air & Space article.  I read it in 'single page' mode, and about half-way thru to the end it got even better.  A similar article on how smaller General Aviation got from the World War 1 era 2-wing designs to single wings would also make for a good read. High with a strut, then high with no strut.  Low, with no wing bracing. T-tail! Tail draggers, then nose-wheel tricycle gear.  Fixed gear, then retractables.  Then Burt Rutan's composite planes, mid-wing pusher engine with front canards that sprouted into other almost tadpole shapes to carry more occupants.

To put it simply, aircraft design is a war of compromises, and when characteristics and configurations of a design are combined in different ways, there emerges some combination that hits a sweet spot.
Like an airliner designed to cruise at high altitude (lots of area + high aspect ratio), near trans sonic speed (lots of sweep), needing 4 engines (best place to put them is on the wings for efficiency and centre of gravity reasons), with the engines on pylons forward (from the discovery by Boeing, during the B-47 program, that the aerolastic flexibility with the rear weight bias of a long skinny swept wing was a problem and the engines could be used as mass balances by putting them on pylons out front to move the wing's centre of mass forward).  And presto!  There's your near universal airliner configuration.
Or take the lowly Cessna 172.  It's configuration satisfies a whole range of requirements, none perfectly, but all just good enough to make it the most practical arrangement, making it the light aircraft of the ages.
My favourite is the DC-3.  A balance of compromises so perfect that over 85 YEARS after it was designed and almost 75 years after it was last built, it is still to this day a viable business tool that people buy, not for show, but to put to work.

The lowly Cessna 172 could be said to have hurt Cessna when they tried to improve the high wing line with the 177.  Cantilevel wing with no struts!, better handling, more aerodynamic front window situated mre forward on the wing for a better view, wider cabin, wider Center of Gravity.  Same 150 HP engine, so maybe underpowered a little.  Ham fisted 172 pilots couldn't land the stabilator tail, or overfueled/overfilled the plane, so after one model year the wing was made more docile and slots were added to the stab, to make the plane tamer, 180HP used.  Reputation hurt sales, limited its life.

I owned a '68 and absolutely loved it.  It was originally planned as the "all-new Skyhawk for '68!".  But cautious heads prevailed and it was decided to keep the Skyhawk as is, except for adopting the O-320 engine, and redesignate the new airplane the 177.  The wing was the cantilever wing from the '66 Centurion, scaled down about 20%.  The stabilator gave it massive tail power and allowed a more forward CG limit.

There were two handling issues.  It had a NACA 63 series laminar airfoil which would develop huge sink rates if you got too slow, and had a sharp stall.  The stabilator had a lot of authority, was geared a bit too fast, and had a lot of rotational inertia with the big mass balance in the fuselage.  Pilots would be afraid of the low speed behaviour and approach too fast.  The stabilator characteristics, especially the fast gearing and high interia, would promote PIOs and badoing badoing badoing down the runway.

Glad that was all fixed in the 1969 model. I've flown a 1973 since 1996, we still like the plane. Just finished putting all new fairings on, replacing all the ABS plastic with fiberglass, plus some speed mods from Aircraft Speed mods.  New avionics too to get caught with GPS moving map world and ADS-B.  Going to be flying it for quite a while more!   http://www.cardinalflyers.com/ for those that want to learn more.

The stabilator slots were the result of the discovery that near full aft stick, possibly while in a sideslip with air curling over the dorsal fin and striking the inboard stabilator at a high AOA, the stabiliator could stall and cause the nose to pitch over right in the landing flare.  The slots just add more AOA stall margin to the stabiliator.  About half the fleet was modified with a field retrofit kit.

The '69 only got the larger engine and the gross weight increase.  It was in '70 with the addition of the constant speed prop that the wing was redone with a fat nose and they reduced the gearing of the stab.  On my '68 I had a Horton STOL kit added with a LE cuff and this tamed the stall behaviour and greatly improved low speed handling.  After the STOL kit I could go anywhere a 172 could go.

Did you get the stab slots too?  I was under the impression "Operation Cardinal Rule" retrofitted those into the '68s.  Lot of '68s were later upgraded to 180 HP engines, some fixed fitch, some controllable pitch.

Mine had them as Cessna sent out retrofit teams to fix the 5 or 600 or so built before the mod.  The 180 conversion cost payload as there was no gross weight increase, worse with CS prop.  '69s are the best; 180 hp with fixed pitch (cheap), higher gross wt so more payload.  Put a Horton kit on it and you have the optimal Cardinal.  Unfortunately only a couple hundred were made that year so they are rare.  The application is harassing me about extended comments so I'd better end it...

Here is another take on this.
Long-distance airliners look remarkably similar because 1) they all use the same propulsion technology i.e., high-bypass turbojets burning kerosene, 2) they all use the same structural materials set (mix of composites and high-strength precipitation-hardening aluminum alloys), 3) they are all subject to almost the same economic constraints (return on assets, interest rates, taxation rates, fuel futures costs, seat occupancy, salary & benefits, etc.) and 4) the laws of physics apply to everyone in the business sector without favoritism.
This yields a pencil fuselage with as many seats crammed in as possible, a 35 degree sweep to the wing, the fewest engines possible, 550 MPH cruise at 35,000 feet.


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Fans of tubofan jet engines
Does the thrust of the fan in a turbofan jet engine decrease proportionally with decrease in the diameter of the fan?
As the fan in a jet engine is responsible for over 70% of the total thrust can a spark ignition engine driven fan of 1 meter give an equal thrust of a 1 meter diameter turbofan jet engine with the jet exhaust thrust removed form the sample? All inlet duct and shroud would be identical, also the fans.

It usually decreases less than proportionallyb because some is made back by higher jet velocity.

There is also an altitude component that effects efficiency.  Lower bypass ratios are preferable the higher you go. Most turbofans are optimized for the airliner's home, 30-40000 feet.  Corporate aircraft like the Global that are designed to cruise above 40000 feet have much lower bypass ratios because the higher you go, it becomes more efficient to move a little air a lot vs a lot of air a little.

The same thing happens at high speeds, since an engine gets very inefficient if its exhaust speed drops below its airspeed.  Hence why you don't see high-bypass turbofans on supersonic aircraft - you see low-bypass turbofans (below Mach 2 or so), turbojets (Mach 2 to 3), or ramjets (Mach 3 and above).

Yeah, and if you go to the other extreme with bypass ratios, you end up with a turboprop.  On the new geared turbofans like on the C series, the bypass ratio is starting to approach turboprop country - stand behind one and you can see that the core is tiny on those engines and they are almost all fan.

You have asked two questions here. The first one:

Does the thrust of the fan in a turbofan jet engine decrease proportionally with decrease in the diameter of the fan?

We need to clarify what you mean by "proportionally". If you mean if you halve the fan diameter, the thrust goes down by half, the answer is no. If you mean if you reduce the fan diameter, the thrust goes down, the answer is yes.
For a subsonic engine, net thrust is given by mass flow (kg/s), and the change in velocity of the mass flow between the inlet and exhaust exit. Exactly how the mass flow and exhaust velocity will change as the fan diameter changes is complex and will vary with each engine, so a general statement cannot be made.
Question two:

As the fan in a jet engine is responsible for over 70% of the total thrust can a spark ignition engine driven fan of 1 meter give an equal thrust of a 1 meter diameter turbofan jet engine with the jet exhaust thrust removed form the sample? All inlet duct and shroud would be identical, also the fans.

Firstly, you have said "the fan in a jet engine is responsible for over 70% of the total thrust" as if this is a fact. It's not - what you can say is something like "in a high bypass ratio engine, the fan can be responsible for over 70% of the total thrust".
So, now to the question part:

Can a spark ignition engine driven fan of 1 meter give an equal thrust of a 1 meter diameter turbofan jet engine with the jet exhaust thrust removed form the sample? All inlet duct and shroud would be identical, also the fans.

If, as you state, the inlet, fan, and exhuast duct are the same, and we ignore the thrust from the core (turbine exhaust flow), then if the fan driven by the spark ignition engine and a fan driven by a turbofan engine are at the same RPM, then, yes, the thrust produced by both fans is the same. The fan doesn't care where the torque required to turn it comes from, so long as the torque is adequate to achieve the same RPM.


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Night VFR in France
I would like to conduct a night VFR in France and I am looking for the special regulations (references) for the following questions

If an airport is approved for night VFR and nothing is mentioned on the VAC map, can I land/takeoff at anytime during the night? How does it work with landing fees?
Can I exit the airport easily if I land after ATS hours?
Is the windsock always illuminated ?
On the Flight Information frequency, is there  always someone available during
the night ?

I think each question should be asked separately (i'm tempted to flag the question as too broad, especially for the 2nd subquestion).

What is the airport you're looking at ?
Some airport tower have a 24h guard, non towered airport you need to ask someone on the ground for authorization.
FIS is in auto-information during night, there's only someone during daytime


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Order of start-up procedure in Piper Dakota PA28-236 between electric pump and mixture
I am flying a Piper Dakota PA28-236. The POH stipulates that for start up the right order is first the electric pump ON and then mixture Rich.
I do not understand the logic behind this. For me, it would make more sense to first put the mixture to rich and then the electric pump ON, in the sense that the fuel could directly go to the cylinders without being blocked.
Could anyone explain the rationale behind this ?

Does the POH state that it must be in this order, or does it just state the steps in an order?  Sometimes there are steps that can be done n any sequence, but some ordering has to be chosen for this printed list.

It states the steps in this order. I have the impression that the vapor lock problem mentioned above might be the answer

The Piper Dakota PA28-236 has a carburetor, not fuel injection. Before starting the engine, with the fuel pump on, and mixture rich, fuel is only going to the carburetor float bowl, not to the cylinders.
The only reason to use the electric fuel pump, before starting, is to insure the carburetor float bowl is full.

The order in which you turn on the fuel pump, or put the mixture to rich, doesn’t really matter as long as both are done before you try to start the engine.
My guess is that Piper decided to put the electric fuel pump first because they felt ensuring the float bowl is full of gas is more important than selecting full rich. (selecting full rich does nothing if there is no fuel in the bowl)
Once the engine is started, the mechanical fuel pump feeds the carb, and the electric fuel pump can be turned off as it is used as a backup only.

thanks for your detailed answer but you are not referring directly to my questions. The question is why the POH indicates during the start up procedure that first you must turn on the electric pump and then set the mixture to rich. I would have expected that first you set the mixture to rich and then you turn the electric pump ON. Thanks in advance for your help

Thanks, I edited my answer with more information and also to add a guess as to why the electric fuel pump is selected on first.

Thanks, what about the vapor lock problem mentioned above? Reading the article I believe it makes sense to first turn on the electric pump, however I agree with you that the impact might also be limited

Vapor lock is more common when starting a hot, fuel injected engine. You will basically never get Vapor lock with a cold, carbureted engine

Following engine manufacturers recommendations are never wrong - they are the people knowing the inner workings best. As to why the suggest a special order, it could be for a specific reason or without reason. I tend to find tha for every recommendation given in a POH not following it has an incident or accident in the history.


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headwind, tailwind, weight, density exponents for DR400/180
I am trying to build a ground roll chart for Robin DR400/180 following the procedure described
http://www.eaa1000.av.org/technicl/takeoff/topaper.htm#References. Does anyone know the tailwind, headwind, weight and density exponent to use for Robin DR400/180. Nothing is published in the flight manual.

It isn't clear what you are asking for.  Three of the variables are external environmental conditions.  A range of weights from zero fuel to MGTOW ought to in the manual.

@MichaelHall if you look at table 1 of the attached link, there are values specific to different aircrafts. I was wondering what are these values for a Robin DR400/180

I don't think that "exponent" is the word you want.  Maybe "coefficient"?

agree, it can be coefficient, I am just re-using the same terminology to help


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Daily use of the UK airspace - VFR flights
I am considering flying to the UK this summer with a small airplane. I heard that the daily use of the airspace is somewhere published by CAA (or NATS), including the activation of Dangerous / Restricted areas. I would like to be able to visualize them via a dynamic map (like the DABS in Switzerland published by Skyguide)
I am however unable to find the exact location of the website. Is there something you can help me with ?

FWIW I know the CAA and NATS websites pretty well, I also cannot find what you're looking for on either.

No affiliation whatsoever, but if you're considering flying VFR in the UK I strongly suggest you purchase a license for SkyDemon.
The annual cost is less than 1 hour flying, and for that you get moving map technology on any phone/tablet, a decent view of the absolute mess that is UK Airspace, as well as exactly what you asked - visualization of all those restricted/danger zones etc.
In addition you get live TAFs/METARs and NOTAMs, reviews of all the airfields by other GA pilots and for a small additional fee some professional airfield plates.
Honestly, there is no way I would fly in the UK (or Europe) without it.

I completely agree on this, Skydemon's visualizations are extremely useful. You can also pay a small amount to file flight plans automatically.

You get I think 5 free (VFR) flight plan with each annual subscription

Oh yes, that's true @Jamiec. I've never needed more than that.

Many thanks all for your answers. I am currently using ForeFlight which also displays the active NOTAM, however I like to benchmark the tool with the official publications to avoid any surprises. If you have any other suggestions, please let me know.


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Does glide ratio improve with increase in scale?
Will the same glider have higher glide ratio if its bigger in proportion or smaller in proportion?

Please allow at least 24 hours before choosing an answer to give all people around the globe a chance to come up with an answer

It depends.  If you simply scale an aircraft up and down, most of the aerodynamics behave the same (or at least approximately so).
In order to get perfect scaling behavior, you need to match Mach and Reynolds number.  This is our clue to the answer.
Gliders will likely be relatively slow, so we can consider them to operate in incompressible flow and we can ignore any Mach effects.
As you increase the aircraft's scale, Reynolds number will increase.  Note that the length term in Re is to the first order.
We'll look at the skin friction coefficient's behavior with Reynolds number.

At high Reynolds number, flow will be turbulent.  In this regime, Cf will decrease as you increase Re (make the aircraft bigger).  Decreasing Cf will mean lower friction drag and improved L/D.
At very low Reynolds number, flow will be laminar.  Likewise, in this regime, Cf will decrease as you increase Re.  So a larger aircraft will mean better L/D.
In the middle, things are complicated.
At intermediate Reynolds number, whether you get laminar or turbulent flow depends on the details of your design.  How smooth is your surface?  Were your airfoils designed for laminar flow?  Is the aircraft maintained in meticulous clean condition?  If the answer is yes, then you can operate on the laminar skin friction coefficient curve -- which is much lower than the turbulent skin friction in this area.
High performance sailplanes are one of the only kinds of aircraft that can successfully operate in this region (i.e. with significant laminar flow when turbulent flow is possible or prevalent).
In this area, increasing size will decrease skin friction coefficient and will improve L/D.  Up to the point where it becomes impossible to maintain laminar flow.  At that point, transition to turbulent flow will occur and the skin friction coefficient will rapidly increase.
Keep in mind, this chart is on a log-log scale.  The X-Axis scale represents four orders of magnitude change in Reynolds number.  To achieve this with scale alone, you would start with a chord of 1cm at Re of 10^4, and each successive vertical line on the chart would represent 10cm, 1m, 10m, 100m.  So the change in aircraft size considered here is enormous!

We're gonna need a bigger boat^H^H^H^Hplane.

Gliders will likely be relatively slow great now i want to see a supersonic glider...

am i off in thinking a 5km wide saucer shaped "mothership" would glide supersonically quite well?

@Michael, https://www.nasa.gov/sites/default/files/styles/full_width/public/thumbnails/image/sts-1_landing_6_sts1-1127-noid.jpg?itok=ZC9RT8Wx

The aerodynamics stay largely the same, and the best glide ratio does not change much. It does, however, occur at a higher airspeed as the aircraft gets larger.
But simply scaling an aircraft up is not really practical. If you scale an aircraft by a factor $a$, the wing area scales as $a^2$, while the weight scales as $a^3$. Structurally, the wing roots needs to be able to hold the entire weight of the aircraft plus its payload times the maximum load factor it is rated for. Since the structural strength of parts of the airframe scale as $a^2$ while the weight is scaling as $a^3$, the aircraft is getting structurally weaker.
Increasing the scale also increases the best glide speed Vg due to the higher wing loading, while decreasing Vno and Vne. The aircraft becomes progressively less flyable as Vg and Vne get closer together. Additionally, the minimum sink rate (which is more important for soaring than the glide ratio) increases due to the higher wing loading and required airspeed to maintain flight.

"If you scale an aircraft by a factor $a$, the wing area scales as $a^2$, while the weight scales as $a^3$" an airplane is not a solid cube, the structural efficiency actually increases with the size: a GA airplane can have an empty weight fraction as high as 0.7 while a big military cargo as low as 0.35 i.e. twice as much structurally efficient despite being 2 orders of magnitude bigger.

@sophit This is true, because larger airplanes are not just scaled-up versions of smaller ones.

Well, I suppose the question is not really clear then...

@sophit  The OP specified "the same glider". Clearly that's not technically possible, but the intent seems clear to me - this is an "all things being equal" type question.

@MikeB: I'd interpret it as a simple "external surface" scale, otherwise the chosen answer wouldn't be correct rather your comment would: "clearly that's not technically possible". But the OP is happy with the chosen answer and that's the important thing

@sophit On the contrary, you can scale an airframe a little bit, and doing so doesn't change the glide ratio much, since it changes the lift and the drag about the same for the same angle of attack and leaves the ratio unchanged. The result ends up being poorly optimized, though.

@Chris-RegenerateResponse this is actually what I meant, the OP is focusing only on the aerodynamic aspect and not on the weight one

@sophit Seems that way. Still seems worthy of mention to me. Also worth mentioning that the glide ratio may not change much but the other aerodynamic quantities like sink rate do change significantly.

@Chris-RegenerateResponse: yep

The glider will have a higher glide ratio if larger in proportion.  The attached illustration is an extract of figure 3.2 in the cited report, below.  Shown is the influence of the scale effect caused by increasing Reynolds number on a wing of aspect ratio 20.  Such an increase is accomplished by increasing physical dimensions of the wing, increasing flight velocity to optimize lift and minimize drag, or otherwise by flight within a dense atmosphere under optimum flight conditions.  One should note that flow over the wing becomes increasingly dominated by viscosity of the atmosphere below a Reynolds number of about 400,000.  At lower Reynolds numbers, aspects of flow separation and flight performance become more difficult to address in a practical way.  The other provided answers give sufficient details for an explanation of these effects.
National Research Council/NAS Publication NMAB-495, Uninhabited Air Vehicles. National Academy Press, Washington, D.C.  2000. 108pp.


text:
What is the top speed of the twin seat Hawker Hunter?
Recently I learnt that the Hawker Hunter had a side-by-side twin seat trainer version. This is unusual, since more often the tandem configuration is used. I'm sure the engineers had a good reason to choose this solution, but it got me wondering: how much is the top speed impacted by the resulting increase in frontal area?
From what I've found online, the specs are the same for both trainer and single-seat versions but that doesn't seem right; if nothing else, the empty weight should be different. Is there a former Hawker Hunter pilot here who trained on the twin seater and can tell us the truth?

The manufacturer
lists over half a dozen variants of Hunter two-seaters.
You probably mean the T.7.
For a fair comparison, its closest single seat variant is the F.6,
maximum speed Mach 0.94
(715 mph) at sea level.
The T.7A, a T.7 with different instruments, has a maximum speed of
Mach 0.92 (690 mph) at 36,000 ft.
So the T.7 is a few per cent slower than the F.6.
But it's no slowpoke: in 1959 it even won
the flying leg of a complicated race against 200 other entries.
Your empty-weight test suggests that these numbers are trustworthy:  the weights posted there say that the T.7 is 762 pounds (5 per cent) heavier.


text:
What naca airfoil is the closest to airfoils used by modern airliners?
I know boeing and airbus use propietary airfoils, but is there a single naca airfoil that is fairly representative of what modern airliners use?

They all start with a rounded bit at the front, get a little bigger and then taper to a point.  Is that close enough?

Modern airliners use supercritical wing profiles, which allow for transsonic airflow speeds over the upper wing surface.
The NACA-profile heyday was a lomg time ago, way before high subsonic jetliners were built. But according to this link there were 8-digit NACA-profiles which were supercritical:

NACA 8-Series:
A final variation on the 6- and 7-Series methodology was the NACA 8-Series designed for flight at supercritical speeds. Like the earlier airfoils, the goal was to maximize the extent of laminar flow on the upper and lower surfaces independently.

The methodology on how to construct the 8-series is given further along in the article, along with a statement that pro’s and con’s of the series are unknown and that they are very seldomly used….


text:
What rules for guiding air traffic should I learn for the FEAST II radar test?
I recently passed FEAST I exams to become an air traffic controller. Next round of tests is a FEAST II radar test.
What general rules used for air traffic guidance should I learn in order to be prepared and easily understand the test instructions?

You should not learn any ATC rules in preparation for the test.

Prior preparation is normally not required for FEAST. Abilities like spatial orientation cannot be learnt. It is true that there are books on test preparation on the market. However, we do not recommend relying on these books as they can give incorrect information about tests or promote inappropriate strategies which may result in negative results in FEAST.

Source
As with FEAST I, FEAST II does not require any previous knowledge. Each test segment is thoroughly explained in advance. Part of what they're testing is your ability to understand and put to use these instructions.
While parts of the test do resemble certain aspects of the job of an air traffic controller, the testing environment is simplified and adjusted to fit the testing needs.
I took the FEAST tests some years ago, and did have a fairly extensive knowledge of ATC in advance. While it probably did help me in some ways, I think it was also a disadvantage in other ways since the test both simplifies and changes some of the ATC rules and regulations.
As I'm sure you're tired of being told again and again, the best way to improve your chances is to show up well rested, well fed and be sure to read the test instructions very thoroughly (and be sure to ask if you are in doubt about anything).
Good luck!


text:
Do modern aircraft still use load bearing ribs in wing construction?
Modern aircraft are generally of monocoque construction. Does this apply only to the fuselage, or are modern wings made without load bearing ribs as well? Are they just for shaping of the metal skin?

I'm sure a structures person can provide a more detailed answer, but yes they carry load. Basically no structure is put on an aircraft that is not load bearing. You can look up the A380 Wing Rib cracking issues for a recent example.

Modern transport category aircraft utilise a semi-monocoque construction. Best to check the FAA Maintenance Handbook Vol. 1, chapter 1 [1]. Basically, both an internal frame and the skin share the load, unlike the older truss-type, where only the frame carried the load, or in a monocoque type where effectively just the "skin" carries the load.
This is even true in a modern composite wing like that of the A350. Here there are metallic ribs, carrying some of the load, with a composite skin [2]. Here is an image of those ribs and the wing structure, and if you check the presentation, you can see the composite skin on the next slide.

[1]: FAA. (2018). Aviation Maintenance Technician Handbook - Airframe Volume 1. Federal Aviation Administration. https://www.faa.gov/sites/faa.gov/files/regulations_policies/handbooks_manuals/aviation/amt_airframe_hb_vol_1.pdf
[2]: Fualdes, C. (2016, 25-30 Sep.). Experience and lessons learned of a Composite Aircraft. [Conference Presentation] 30th Congress of the International Council of the Aeronautical Sciences ICAS, Daejeon, Korea. https://www.icas.org/media/pdf/ICAS%20Congress%20General%20Lectures/2016/2016%20Composite%20Aircraft%20Fualdes.pdf

Your question contains the answer ;-)
Since ribs have the really important job of preserving the aerodynamic shape of the wing, it follows that they are stressed. They are actually so stressed that in some design they look more like a truss then a thin layer of aluminium, see for example this cutaway of the Concorde:

Ribs are also used to introduce big loads in the structure: engine's pylons are attached to ribs; external weapons or storages are attached to ribs (these attachments are called hard-points); the hinges of flaps, slats and control surfaces are also attached to ribs.
Ribs are also used to delimit fuel tanks.

Somehow you need to carry the lift chordwise to the spar, where it is needed to keep the fuselage up in the air. Doing this with a rib is structurally much more efficient than beefing up the skin to do this without ribs. Also, flap, gear and engine loads will need ribs because the wing skin by itself would buckle and fail from those concentrated loads.
Even lightly loaded wings like those of composite gliders use ribs in their wings, albeit only a few. One at the root is needed to carry the lift loads from the lift pins to the spar, others to keep the water ballast tanks in place (or to form the water ballast tank in a "wet" wing).


text:
Why did wings start to be made with aluminium skin instead of fabric?
Why did engineers start designing planes with aluminium skin on the wings instead of fabric covering? While the force of lift acts on the skin, to my knowledge it is not subject to high stresses

Wings and fuselages are subjected to twisting loads as well as bending so they need to be torsionally stiff.  Prior to aluminum, the options were to make the wing/fuse with a frame of spars and ribs with fabric to provide the aerodynamic contours, braced with struts and/or wires to provide the torsional stiffness as well as resistance to bending, or, to do away with the struts and brace wires and make a monocoque structure (like an eggshell) with a plywood skin to provide torsional stiffness (with the spar beam inside for bending).  If you wanted to go fast, you had to do away with all the bracing so plywood monocoque was the way to go.
When "duraluminum" came along after WW1, it was realized that as well as replacing sitka spruce for spar beams and ribs, it could also replace plywood for a stressed skin monocoque structure.  You could build an airplane in the 1930s using plywood stressed skins bonded to wood ribs and spars, or aluminum stressed skins over aluminum spar beams, and have pretty much the same performance.  You could make your egg about of plywood, or aluminum sheet, and it was pretty much the same egg.
There were so many manufacturing and durability advantages to aluminum (no glue, no wood rot etc etc), it was a no brainer to switch, once the metallurgy started to become understood well enough to use it as an engineering material, and plywood skinned wooden airplanes faded quickly.

Also fabric, and particularly the fabrics available before modern synthetics, deteriorate when exposed to sun and air.  Build from aluminum, and you don't have the expense of recovering it every few years.

Spot on! As a structural material aluminium and wood are surprisingly similar per unit of mass.

These bays carbohydrate fibre composite (plywood) has been replaced with carbon fibre composite,.

You missed the intermediate step.  Glass replaced cellulose in the 50s.

I would assert that the breakthrough occurred when aluminum became (relatively) inexpensive to extract from ore on industrial scales. There was also a psychological factor, in that the popular american football coach Knute Rockne died in the crash of a Fokker passenger plane with a wooden wing when it came apart in a storm. All-metal construction then became a selling point for airlines.

Aeroelastic flutter is an issue at higher speeds and the simplest way to overcome that is to use stiffer materials.

That's not the reason. Plywood monocoque structures are just as stiff as aluminum ones.  Aluminum replaced plywood in airplanes for ease of manufacture and durability.

@JohnK I see what you mean but that would technically be a reason to switch from wood to aluminum rather than fabric to aluminum (as well as rot). It would mean that there is a gap in the OP's question that they were not aware of.

@John K: Are there any aluminum frame planes with fabric skin?

@DKNguyen yes the question is kind of incomplete.  My bringing plywood into it was to complete the picture and to point out that what he really needed to compare is plywood monocoque vs alum monocoque when talking about the adoption of aluminum in the 1920s, not fabric covering per se.  There are aluminum wing airplanes with stressed skin AND fabric covering. The Ercoupe has an alum cantiliver wing but the stressed skin only is applied around the leading edge to make a D torque box..  The aluminum rib bays behind the spar are open and covered with fabric.

@Aaron Holmes Lots of light planes have wings with aluminum ribs and spars with fabric. The aluminum is just replacing the spruce plank of the spar and the wood stick ribs. The wings require bracing same as a wood one.  Later models of the CItabrias have alum spar and alum ribs with strut braced wings and fabric.  Early Luscome 8As were the same.  The wings are considerably lighter than the later full aluminum skinned wings even though they eliminated the aft lift strut .

@John K: Thanks for the side answer. That's really interesting.

Principally to carry greater loads.  The skin on the wing forms an integral part of the wing box, which provides greater structural rigidity.  It also provides better aerodynamic characteristics under loads. For instance Boeing has a particular way that it designs its wing scans, which is a proprietary secret, that causes them to flex when under load for better aerodynamic qualities.

Redundant sentence is redundant.

that's because it came from the department of redundancy department.

"It also provides better aerodynamic characteristics 100 loads." What is "100 loads"?

I saw it.  Goddamned autocorrect.


text:
What's the relationship between mach number and drag force on an airplane?
From what i've read drag coefficient at mach 1 is several times (up to 10x) the drag coefficient below drag divergence mach number. It drops as the mach number increases.

It's easy to find plots representing experimental data for Cd vs M, but I wonder what's the relationship between drag FORCE and mach number. I wasn't able to find a plot for this. I wonder if an aircraft that can reach Mach 1 can automatically reach say mach 1.2, given that even though Cd drops the drag still increases with the square of speed (and engine power required is cubed)

The plot you show is typical for airplanes which are not designed for supersonic flight. Yes, there is a maximum in the drag coefficient around or slightly above Mach 1, but in a properly designed airplane this maybe triples the subsonic drag coefficient and shows only a slight reduction as Mach number increases further. Take the F-16, for example: Here, the drag coefficient is nearly constant above Mach 1.1.
Your observation that due to the increase in dynamic pressure the drag force will increase with Mach number is correct. Some early designs with heavy and less powerful jet engines would just break the sound barrier but not even reach Mach 1.2. However, normally the pilot would keep dynamic pressure nearly constant and climb as Mach number increases. This, of course, makes the airplane fly at constant dynamic pressure so the drag force would indeed go down with increasing Mach number, but not as dramatical as in your plot.

F-16 drag coefficient from Ray Whitford's Fundamentals of Fighter Design lecture

@ymb1 Details are hard to come by. The best I have is a sketch straight from Ray Whitford's Fundamentals of Fighter Design lecture.

I believe AGARD CP-242 has a detailed plot of the CDmin of F-16.

The plot you show is actually typical for rockets or missiles. Here's the CD0 for AIM-9 at sea level:

As for F-16 and Mirage-2000, you'll find a pretty flat curve above M1.0 as the Cx0 and CD0 (measured with a difference in alpha) stays almost constant. But as already indicated by other answers, Total Drag = CD * dynamic pressure * Sref, you might want to pick an altitude to convert mach number to dynamic pressure. But usually the plot already has a testing altitude for it.

The source for F-16 is probably from AGARD CP-242. I can only find a translated one. CDmin and flight test points are for 30,000 Ft.

Aerodynamic drag is described in the form of a product of:

Moving body shape parameter $C_D$
Dynamic pressure q.
Reference area.

For incompressible flow, dynamic pressure can be taken as $\frac{1}{2} \rho V^2$, with $\rho$ = air density. For compressible flow, dynamic pressure depends on the local speed of sound : q = $\frac{1}{2} \gamma p M^2$, with p being static air pressure and $\gamma$ = ratio of specific heats = 1.4 for air at sea level.
So the drag equation becomes $$D = C_D. \frac{1}{2} \gamma p M^2. S$$
Engine power cubed is of very little relevance in this situation. High subsonic and transsonic aeroplanes are powered by jet engines, which deliver exhaust thrust.

The amount of drag is related to the coefficient of drag by the following formula:
$$D = C_d (\frac{\rho V^2}{2}) A$$
Where

$D$ is the drag
$C_d$ is the coefficient of drag
$\rho$ is the density of the air the aircraft is moving through
$V$ is the velocity of the aircraft
$A$ is the reference area of the aircraft.

That last value may require some explanation. Drag is typically measured directly, such as in a wind tunnel, rather than being calculated. It's the coefficient that's usually calculated, and to do so, you need to pick an area. You can use the frontal area of the aircraft, the total skin area, or certain other areas. Each choice of reference area will yield a different value for the $C_d$, so to get from the coefficient back to the actual drag force, you need to know the reference area that was originally used to go the other way.


text:
TIS-B anonymous address
I could not understand what is the difference between ADS-R with anonymous address (CF=6, IMF=1) and TIS-B CF=5, which on ICAO Doc 9871 is described,
for Version 1 as:

Fine TIS-B Message AA field contains a non-ICAO 24-bit address

and for Version 2 as:

TIS-B messages that relay ADS-B Messages using anonymous 24-bit
  addresses

Any specific example when CF=5 is used?
Thanks!

My question was due to the confusing mention of ADS-B in what was supposed to be either TIS-B or ADS-R.
I've now confirmed that the text on ICAO Doc 9871 Version 2 I quoted was due to a error.
In fact DO-260B, and even Doc 9871 Version 2 at table C-37 specify that TIS-B CF=4 is "Fine TIS-B message, AA field contains a 24-bit anonymous aircraft address".
ICAO e-mailed me confirming that this has been fixed into their working document.

it might be useful to quote a source for your discovery.  Thanks!

@bclarkreston thanks. I added further details.

ADS-R is only used for relaying ADS-B messages from one datalink to another, e.g. 978UAT to 1090ES. This is currently only applicable to the US since no other country has adopted 978UAT.
TIS-B is used when a ground radar station transmits ADS-B messages on behalf of a non-ADS-B contact. If the contact has Mode S, the ground station will use the contact's Mode S address, which may be either assigned or anonymous. If the contact does not have Mode S, the ground station will generate a trackfile address, which is the 12-bit Mode A code plus 12 locally-assigned bits.

TIS-B is a service that broadcasts traffic information. This traffic information is gathered by using various surveillance sensors such as ADS-B and/or radar.
In case the surveillance source is a Mode A/C radar, and no ADS-B track can be correlated to the radar track, no 24 bit address is known for the aircraft. Now the TIS-B message will be filled with a 24 bit address that is created ad-hoc and temporarily assigned to that radar track. This is not a standard ICAO address (which is assigned through official procedures of the ICAO member states) but instead a (more or less random) number is used.
To clearly distinguish between the ICAO 24 bit addresses and the ad-hoc addresses (which may be accidentally the same) a different message indicator is used in TIS-B.
A similar scheme is used for ADS-B and ADS-R. In case an ADS-B transmitter cannot be assigned an ICAO 24 bit address (e.g. because the country only assigns addresses to aircraft and the transmitter is on a car or a fixed obstacle) an "anonymous address" can be uses. In that case a special flag is used to avoid confusion between  ICAO addresses and anonymous addresses.

I think your answer explains what is the use for non-ICAO/anonymous address, which is very obvious, but the question is instead what is the difference in both the frames I mentioned (ADS-R with anonymous address and TIS-B with non-ICAO/anonymous address).


text:
Will Thrust on Rotor (Main) increase with altitude?
Can some one please let me know,if the thrust required on the rotor head increases with the increase in altitude in an Helicopter or decreases with increase in Altitude?.
Thanks in advance,
Pawan

For no vertical acceleration, lifting force (i.e. thrust, as you use the term) = weight. That's independent of altitude and everything else. Now, the power required to gererate that lifting force increases with altitude, which is why helicopters have maximum altitudes for In-ground-effect (IGE) and Out-of-ground-effect (OGE) hover. Those ceilings vary with weight & temperature.

In case you would need a strict answer in addition of @Ralph comment, weight and thrust will slightly decrease with altitude due to the gravity gradient (-0.35% at FL350, though there are few helicopters at this altitude)

I presume that OP means torque on the rotor, since thrust must always compensate weight to maintain altitude. Thrust T, torque Q and power P equations for momentum theory in the hover:
$$T = C_T \cdot \rho A (\Omega R)^2$$
$$Q = C_Q \cdot \rho A (\Omega R)^2  R$$
$$P = C_P \cdot \rho A (\Omega R)^3$$
with $C_Q$ being identical to $C_P$.
At any altitude, thrust must equal weight. Air density $\rho$ decreases with increasing altitude, so $C_T$ must increase with increasing altitude which it does linearly.

Above graph from Leishman shows the relationship between $C_P$ and $C_T$, which can be approximated as:
$$C_P = \frac{\kappa \cdot {C_T}^{3/2}}{\sqrt{2}} + \frac{\sigma \cdot {C_D}_0}{8}$$
with $\kappa$ = loss factor and $\sigma$ = blade solidity.
So power required increases with $T^{3/2}$, and therefore increases with increasing altitude. Since $C_P$ = $C_Q$, torque does as well. Note that the weight at altitude has decreased due to the fuel burn required to reach higher altitude, which will compensate some of the additional torque required.

The thrust required from a main rotor to keep the helicopter aloft remains essentially the same regardless of altitude. Thrust must equal weight to keep it aloft. Since you mentioned thrust, we're talking about hovering and not forward flight (where airflow over the spinning rotor disc produces lift).
The thrust produced by a main rotor, assuming a constant torque and blade angle of attack, decreases with altitude. The air gets thinner, but the blades aren't moving any faster or getting any larger.
In the case of a helicopter, the blades can't turn any faster. They're already at the optimal speed, any faster and either the tips go supersonic and lose a lot of efficiency, or the blades depart from the aircraft... definitely Not Good.
Nor can rotor blades become larger to handle higher altitudes without adversely affecting low altitude flight with increased drag and mass.
So the angle of attack of the blades is increased to keep the thrust sufficient... up to a point. Over a certain altitude, can be FL150-200 depending on the particular model, even that won't restore thrust to where it needs to be... the AOA will increase to where the blades stall.
Note that tilt rotor aircraft cruise at altitudes much higher than helicopters, because they are using higher speed and airflow over the wings to produce lift, not downward thrust from the rotors.


text:
What is a "confidence maneuver"?
Order JO 7110.65T Section 6 Part 5-6-1 says

Vector aircraft:
  a. In controlled airspace for separation, safety, noise abatement, operational advantage, confidence maneuver, or when a pilot requests. Allow aircraft operating on an RNAV route to remain on their own navigation to the extent possible.

What is a confidence maneuver?

As quoted from the entry in the Pilot/Controller Glossary:

CONFIDENCE MANEUVER− A confidence maneuver
  consists of one or more turns, a climb or
  descent, or other maneuver to determine if the pilot
  in command (PIC) is able to receive and comply with
  ATC instructions.

This could be useful if the controller suspects a hijacking, hypoxia, radio failure, or other situations that would compromise reception and execution of ATC instructions.
Perhaps the best example of an implementation of this would be when ATC suspects a partial aircraft radio failure; a confidence maneuver vector might enable ATC to determine that the pilot was still able to receive instructions, though aircraft radio transmission might be compromised.

In the case of attempting to troubleshoot a communications issue only, a simple "N12345, if you hear this transmission, ident" is easier and quicker. Otherwise I agree completely.

@randomhead Yes, assuming that the aircraft is so equipped. That technology is new, relative to the introduction of the Confidence Maneuver. Most aircraft now flying in the US do have transponders, but many do not, especially those without electrical systems. By it's very nature the Confidence Maneuver is rare.

That's not correct. JO 7110.65T CHG 2 (3/10/2011): This changes adds “Confidence Maneuver” as a reason for issuing a vector. This change cancels and incorporates N JO 7110.534, Confidence Maneuver and No Radio (NORDO), effective October 1, 2010.

You may be right about this particular application of the term confidence maneuver to ATC vectoring. The concept of a confidence maneuver from a pilot perspective has been around for decades, at least dating back to the early 70s.


text:
What wind change gradient is necessary to be classified as wind shear?
Wind shear is defined as a sudden change in the speed or direction of the wind with altitude.  There's got to be some ratio of speed to altitude or directional change to altitude beyond which it's defined as shear, but I can't figure out what it is.
What is the actual change in wind direction or speed over altitude for a wind shift to become considered wind shear?

Are you looking for high-altitude wind shear, low-altitude, or both types?

The change in wind speed and/or direction can also occur horizontally in a wind shear.

On one hand this has to do with how wind shear is actually reported which makes it tough to put an official value on it some times. Wind shear is often reported by pilots when they experience it on landing, takeoff or in the air. This may come in as a PIREP and be disseminated out to other pilots. When you are actually in the left seat you will see the manifestation of wind shear as a sudden loss of airspeed or severe weathervaning of the aircraft. There are many conditions that may cause a pilot to report wind shear. Since different planes at different times may experience different conditions its somewhat subjective.
The Wikipedia definition puts it at:

Airplane pilots generally regard significant wind shear to be a horizontal change in airspeed of 30 knots (15 m/s) for light aircraft, and near 45 knots (22 m/s) for airliners at flight altitude. Vertical speed changes greater than 4.9 knots (2.5 m/s) also qualify as significant wind shear for aircraft.

The FAA has a nice briefing on it here and their definition (as official as its going to get) is:
Frontal Wind Shear

As with so many things associated with weather, there is no absolute
  rule, but a couple of clues tell you that wind shear may occur:

The temperature difference across the front at the surface is 10 °F
  (5 °C) or more.
The front is moving at a speed of at least 30 knots.

Thunderstorm Wind Shear

These winds can change direction by as much as 180 degrees and reach
  velocities of 100 knots as far as 10 miles ahead of the storm. The
  gust wind speed may increase by as much as 50 percent between the
  surface and 1,500 feet, with most of the increase occurring in the
  first 150 feet.

Temperature Inversion Wind Shear

One particularly bothersome aspect of temperature inversion shears is
  that as the inversion dissipates, the shear plane and gusty winds move
  closer to the ground. In some areas of the Southwest, a 90-degree
  change in direction and 20- to 30-knot increases in surface winds in a
  few minutes are not uncommon

Wind Shear From Surface Obstructions

Some airfields are close to mountain ranges, and mountain passes are
  close to the final approach paths. Strong surface winds blowing
  through these passes can cause serious localized wind shear during the
  approach. The real problem with such shear is that it is almost
  totally unpredictable in terms of magnitude or severity

From personal experience low level wind shear can really do a number on small planes. Bigger planes with more mass will still be affected but less so from what I hear. I have seen speed drops of 15kts momentarily and heading shifts upwards of 30 degrees on final approach in a Piper Warrior in even light shear conditions.
Here is an overly dramatic (and personal favorite) rendering of a micro burst causing wind shear

(source)
Some airports (110 in the US so far) have been lucky enough to have Low Level Wind Shear Detection Systems installed which are pretty neat units, they will detect that as:

LLWAS wind shear alerts are defined as wind speed gain or loss of between 20 and 30 knots aligned with the active runway direction. "Low level" refers to altitudes of 2,000 ft (610 m) or less above ground level (AGL)

Here is a pretty neat overview from NASA of the various methods to detect it in the air.

To be fair, that rendering is of a micro burst (which causes wind shear), not of the general case wind shear. On a different note, as far as I understand it, while the wind speed difference would proportionally more severe for a smaller aircraft, wouldn't wind shear have more effect on larger aircraft due to momentum maintaining the air speed difference for longer than with a smaller aircraft?

The way I understand it since the shifts are so fast the large planes simply cant be altered as fast and see less of change. Their momentum keeps them in place really. Again this also depends on control surface area and other factors.

You are correct that momentum "keep the planes going", but your conclusion is not. The momentum maintains the same speed relative to the old air; i.e. the new air is not able to accelerate the airplane as quickly as a small airplane, thus the airspeed drops (or increases, whichever way the wind changes). A small airplane has less mass and is quicker to adjust to the new air, thus sees the change for a much smaller amount of time. If the change is gradual enough, the small plane might not see a change at all.

'Wind shear' isn't defined by a certain gradient, but by the fact that there is a change in wind speed or direction (not necessarily with altitude). When flying through a wind shift, you experience wind shear. The strength of the turbulence experienced when flying through changes in wind varies with the exact situation and size of the aircraft.
From the Pilot/Controller Glossary:

WIND SHEAR - A change in wind speed and/or wind direction in a short distance resulting in a tearing or shearing effect. It can exist in a horizontal or vertical direction and occasionally in both.

Technically, 'short distance' could imply gradient, but for all practical purposes that is over-thinking the definition and I don't think this line exists.
As voretaq7 notes, 10kts of wind fluctuation within 2000 AGL is defined as an Urgent PIREP. There is not a time-window though, so a sufficiently loose definition of instantaneous is assumed.
So to answer the question: one that you can feel.

Wind shear itself is a gradient.

@casey Yes, that's the point. The fact that there is a gradient means wind shear. Any gradient that's not zero is a change.

asking for the gradient is technically the same as asking for the definition of "short distance".

Note that there are lines of demarcation within "wind shear" - specifically, PIREPs for Wind Shear are considered "Routine" if an airspeed variation of <10 knots is noted, and "Urgent" if the airspeed variation exceeds 10 knots.